Currently, Power Over Ethernet (PoE) is standardized by IEEE 802.3af and 802.3at. In PoE, Power Sourcing Equipment (PSE) is connected to a Powered Device (PD) via a standard Ethernet cable, and the PD is supplied DC power over the Ethernet data lines in the cable. In this way, the need for providing any external power source for the PD can be eliminated. The IEEE standard provides various hand-shaking requirements, voltage requirements, current requirements, power requirements, and other requirements for standardization.
In order for the PSE to know if the PD is PoE-enabled and the maximum power level of the PD, a low voltage/current detection and classification routine is conducted between the PSE and the PD. The PSE and PD contain controller ICs that carry out the various PoE routines and provide the required identification information. After a successful hand-shaking routine, the PSE supplies the full PoE voltage, such as 44 volts, to the PD. The PD controller IC detects the PoE voltage and then issues a Power-Good (PWRGD) signal, which closes a switch to connect the unregulated PSE voltage to a PD voltage regulator, such as a switching voltage regulator using transformer isolation. The switching regulator may be a buck/boost flyback regulator or any other suitable regulator. The regulator then supplies the target regulated voltage (e.g., 5 volts) to the PD load, such as electronic circuitry.
To achieve higher reliability that a PoE voltage is provided to the PD, the PD can receive two independent PSE channels. The channels can be provided via two Ethernet cables connected to two independent PSEs, or the data pair and spare pair of wires in a single Ethernet cable can be used to couple the two PSE channels to the PD.
FIG. 1 is a conventional design for a PD 10 with redundant PSE inputs, typically from an Ethernet switch. Each input channel, PSE CH. 1 and PSE CH. 2, is connected to a separate PSE power supply (not shown) and PSE controller IC (not shown) in the associated PSE (not shown). Since the incoming PSE voltage can be either polarity, a diode bridge 12 and 13 is provided for each channel. The PSE side also may supply differential data over the same twisted wire pair that supplies the DC power to the PD. The data may be tapped off at the inputs of the diode bridges 12/13 and applied to the PD load.
The PD controller IC for each channel is identified as PD controller 14 and PD controller 16. Upon powering up of the system, the PSE controller for each channel conducts a low voltage/current detection routine to detect the presence of a 25 k ohm resistor across the data wires, signifying that the PD is PoE-enabled. The resistors may be part of the PD controller ICs or external. If the PSE detects that the PD is PoE enabled, a low voltage/current hand-shaking routine is performed to obtain additional information about the PD and PSE, such as whether the PD is a Type 1 or Type 2 device, specifying different maximum power requirements and related requirements. This is the classification phase of the hand-shaking. The PSE low voltage powers the PD controllers 14 and 16 during this time. Each of the PD controllers 14 and 16 may operate independently during this phase.
Once the hand-shaking routine has been successfully performed, the PSE for each channel supplies the full voltage, such as 44 volts, to the input channels of the PD 10. The PD controllers 14/16 sense the proper high voltage and issue a Power-Good (PWRGD) signal, which closes the associated series switch 18 and 20 to couple the unregulated 44 volts to the input of a power supply 22 and 24 for each channel. The power supply 22/24 may be an isolated switching regulator using a transformer and isolated feedback (transformer or opto-coupling). As an example, each power supply may regulate the PSE voltage to 5 volts for powering the PD load 26. The load 26 may be electronic circuitry or any other load.
Diodes D1-D4 are coupled between the outputs of the power supplies 22/24 and the PD load 26 so that current from one power supply is not coupled to the other power supply. If the channel 1 voltage is higher than the channel 2 voltage, diodes D1 and D3 will be forward biased to couple the power supply 22 output to the PD load 26. Conversely, if the channel 2 voltage is higher than the channel 1 voltage, diodes D2 and D4 will be forward biased to couple the power supply 24 output to the PD load 26. If the voltages are precisely equal, both power supplies 22/24 may supply current to the PD load 26.
The advantage of the design of FIG. 1 is that there is no delay in providing power to the PD load 26 if one of the PSE channels fails. This is referred to as hot standby. The downside of the design of FIG. 1 is that two power supplies are needed, which adds cost, size, and power consumption. Further, if one channel is not supplying current to the PD load, and the unused channel draws less than 10 mA (an IEEE 802.3 standard), the PSE for that channel will interpret this as being a disconnected PD and will cease supplying voltage to the PD. In that case, hot standby is lost. However, an operating switching regulator may draw at least 10 mA even when the load is uncoupled from its output.
As an alternative, the two PD channels can be controlled by separate PD controller ICs (as in FIG. 1), and the two unregulated sets of PSE voltages can be connected via a diode bridge to the input of a single power supply (e.g., a switching regulator). In that way, only the higher of the two PSE voltages is coupled to the power supply input. However, according to the IEEE standards, the PSE must detect a minimum current drawn by the PD or else the PSE will cease supplying the full voltage. This minimum DC current is typically 10 mA and is called a PoE Maintain Power Signature (MPS). The PD must also provide an AC impedance of less than 26.3 k ohms in parallel with 0.05 uF. Disadvantages of this approach include: 1) the PSE for the unused channel will shut down since the minimum current requirement is not met; 2) the unused channel will continually cycle through the PoE detection/classification sequence; and 3) the system is not a hot-standby system since it takes time to power up the unused channel after a failure of the other channel.
What is needed is a PoE technique that provides hot-standby for the PD load but does not have the drawbacks of the prior art.